Department of Computer Engineering
University of California at Santa Cruz
Image Acquisition
Hai Tao
Department of Computer Engineering
University of California at Santa Cruz
Human visual system
The main components of the eyes (courtesy of Forsyth and Ponce, reprint from
Tompson et al., 1966)
Department of Computer Engineering
University of California at Santa Cruz
Human visual system
Rods and cones. Left: the distribution of rods and cones across the retina. Right: (a)
cones in the fovea; (b) rods and cones in the periphery (courtesy of Forsyth and
Ponce, reprint from Wandell, 1995). Blind spot - ganglion cell axons exit retina. In
the fovea, the density of cones reaches 1.6x105/mm2. Cones – less sensitive to light,
but more nerve connection and high spatial resolution. Rods – very sensitive to light,
but with low spatial resolution because many rods connection to a single neuron.
Department of Computer Engineering
University of California at Santa Cruz
Pinhole cameras
Q
Q
Q
Q
For a ideal pinhole camera, a single ray from any scene point reach the
image plane to form an inverted image. Therefore the image is sharp
The same image can be obtained on a virtual image plane in front of
the pinhole
The pinhole 3D-to-2D projection is called a perspective projection
Small pinhole allow small amount of light to pass through, real
cameras use sophisticated optics to overcome this problem
Department of Computer Engineering
University of California at Santa Cruz
Sensing
Q
CCD cameras – charge-coupled-device
• Rectangular grid of electron-collection sites made on the wafer
• Photons strike the silicon and the electrons are generated, which
are collected over a fixed period of time
• Charge coupling: change packets are transferred from site to site
• Output one row at a time
(courtesy of Forsyth and Ponce)
Department of Computer Engineering
University of California at Santa Cruz
Lens distortion
Q
Q
Q
Real length can be better modeled using thick lens model
Geometric distortion caused by difference in focal length
at different parts of the lens
Barrel distortion can be modeled using simple radial
function with the relation 2
4
x = xd (1 + k1r + k2 r )
y = yd (1 + k1r 2 + k2 r 4 )
where r 2 = xd2 + yd2
pd
p
o
o
Nominal image
Barrel distortion
Department of Computer Engineering
University of California at Santa Cruz
Lens distortion
Q
Example
Original image with lens distortion
Corrected with k1=0.11, k2=0.019
(courtesy of Shawn Becker)
Department of Computer Engineering
University of California at Santa Cruz
Still cameras and video cameras
Q
Still cameras
• Film cameras – effective resolution is approximately 6,000 by
4,000 pixels
• Digital cameras – 1M pixels (low end), 3-3.5 M pixles (midrange),
better optics and resolution (high end), price is dropping
Q
Video cameras
• Analog video camcorders (VHS, Hi8, etc), need frame grabber
(internal PCI board or external converter) for digitization.
(resolution 640x480 or 720x486 depends on frame grabbers)
• Digital video camcorders (resolution 640x480 )
- Store on digital media such as Mini DV
- Output digital video - USB 1.1 12 Mbps, FireWire 400Mbps,
USB 2.0 480 Mbps
• Web cams – low image quality, low price
• Industry video cameras – various specs
Department of Computer Engineering
University of California at Santa Cruz
Acquiring high quality images and videos
Q
Main elements of a camera
• Lens – controlled by focusing ring and zoom
• Aperture – controlled by aperture ring
• Shutter – speed control
Q
Goals
• Find the combination that achieves sharpest images for the largest
range of scene with correct exposure
lens
(reprint from The Complete Kodak
Book of Photography)
aperture
shutter
lens aperture
shutter
Department of Computer Engineering
University of California at Santa Cruz
Focus
Q
Q
Manual focus and auto
focus
If focused at close
distance, only a shallow
part of the scene appear
sharp.
(reprint from The Complete Kodak Book of Photography)
Department of Computer Engineering
University of California at Santa Cruz
Shutter speed
Q
Q
Q
The amount of time a film/CCD is exposed to the light
A slow shutter speed causes motion blur
1/60 or 1/100 is good for common tasks
1/30 sec
1/125
(reprint from The Complete Kodak Book of Photography)
1/500
Department of Computer Engineering
University of California at Santa Cruz
Aperture
Q
Q
Q
Opening of the length, usually has the scale of f/1.4, f/1.4, f/2, f/2.8,
f/4, f/5.6, f/8, f/11, f/16, f/22.
The numbers get bigger, the aperture opening gets smaller
Smaller aperture means sharper image (pinhole camera)
f/2
f/5.6
(reprint from The Complete Kodak Book of Photography)
f/16
Department of Computer Engineering
University of California at Santa Cruz
Tips
Q
Q
Unwanted camera motion during the exposure can be
reduced by holding the camera still or using a tripod
Using lighting equipment to achieve small aperture+fast
shutter speed